Electrical submersible pumps (ESPs) are used in the geothermal, oil and gas and water wells for producing fluids from the subterranean well. Traditionally, subterranean wells are completed in porous formations having naturally high permeability and which contain water, oil, natural gas, heated water, brine and/or steam in relative close proximity to the surface of the earth. Geothermal wells are also completed in low permeability formations that contain little to no geothermal fluid. For these low permeability formations, the permeability of the formation is engineered or enhanced through stimulation methods such as pumping of cold water to generate fractures within the formation. This creates or enhances a geothermal reservoir in the high temperature formation to enable development of an Engineered or Enhanced Geothermal System (EGS).
Currently, ESP systems are not suitable for most high temperature applications, especially geothermal applications. ESP systems are susceptible to pump cavitation due to boiling in high temperature wells producing water and/or brine above 100° C. The temperature of the earth grows hotter with increasing depth, and geothermal systems can have well temperatures ranging from 150° C. to greater than 300° C. Advanced methods for recovering heavy oil may involve the use of steam to mobilize or heat oil and water produced from the reservoir having a temperature above 200° C. ESP systems used to recover oil with hot water in these steam flood wells are exposed to temperatures above design limits of current ESPs.
ESPs are comprised of two main parts, an electric induction motor and a centrifugal pump. The electric motor is used to drive the pump. The motors and pumps both have small aspect ratios (diameter to length ratio), typically 5-10 inches in diameter and 10-30 feet long. The pumps are used down-hole in oil-field applications to pump oil from reservoirs to the surface. The ESP is placed in an oil well typically hundreds to thousands of feet underground. Oil producers have been using ESPs in Steam-Assisted oil-field applications, where the motors and pumps are operating in reservoirs with temperatures exceeding 400° F. As a result of heat generated on the interior of the motor (due to electrical and windage losses) during operation, the interior of the motor may reach temperatures significantly hotter (between 50°-100° F.) than the reservoir temperature.
ESP manufacturers all produce a line of ‘high temperature ESPs’ that are specifically designed to operate in high temperature environments. The design enhancements used in the current state of the art high temperature ESPs primarily focus on material selection (epoxies and insulation) in the motor, so that the electrical components can operate at elevated temperatures. Despite these design enhancements, thermal failures of ESPs are still a significant cost to oil production companies and a significant portion of total production is at risk from ESP failure.
Empirical evidence shows a strong correlation between a reduction in motor operating temperature and increased run life. Empirical evidence from the industry suggests that a 20° F. reduction in peak motor temperature could result in a 50% increase in run life. For ESP motors during operation, the interior components of the motor typically operate at temperatures 50-100° F. higher than the surrounding reservoir temperature. However, if a downhole cooling device (e.g., a refrigerator) can be used to provide a low temperature heat sink downhole, and depending on the capacity of the refrigerator, the internal components of the motor could be cooled to the reservoir temperature or even lower, with proportionate increases in run life.